What Sivir is lacking
I believe Sivir is in need of a slight tweak. Every ADC in the game (well most) have burst damage with their ultimate that allows them to finish enemies off at a distance. Sivir's ultimate adds mobility and attack speed but does not give the burst damage that most ADC have in their kit. Even some ADC that dont have an immense amount of burst damage with their ult have some pretty good poke (like kog'maw's R) that again Sivir doesnt have. Sivir does some things really well (like wave clearing with her W) but her lack of a finisher puts her behind the rest of the Champs that play bottom (even Ziggs). Her R almost seems like an ability that would not be an ultimate (if a little weaker). Please think about this in upcoming patches.